A conventional photoelastic method is a real time, whole field, highly sensitive and non-destructive method that determines stress distribution in a birefringent material.
There are two kinds of birefringent material, permanent birefringent material and temporary birefringent material. The permanent birefringent material is a material that has birefringence permanently. The temporary birefringent material is a material that exhibits birefringence when it is loaded and the birefringence disappears when the material is unloaded.
Birefringence is the decomposition of a ray of light into two rays when it passes through the birefringent material, depending on the polarization of the light.
Polarization is a property of waves that describes orientation of their oscillations. Electromagnetic waves, such as light and gravitational waves, exhibit polarization.
U.S. Pat. No. 5,400,131 discloses a method for measuring stress in an object of birefringent materials. The stress has both magnitude and direction. The method comprises steps of passing polarized light of first, second and third wavelengths through an object and an analyzer to produce respective fringe patterns, measuring and recording intensities of light for each wavelength emitted from the analyzer for multiple positions in the respective fringe patterns and combining the recorded intensities for the first, second and third wavelengths to form a ramp map having discontinuities at predetermined values of stress and converting the ramp map to a stress map indicating the magnitude of the stress in the object as a function of position within the object, which cannot quantify residual stress precisely because each predetermined value comes from a different source.
Accordingly, a new apparatus and method are needed to quantify residual stress of a birefringent material.